At present, DRAM and flash memory are the most frequently used memory devices in the electronics industry. Particularly, the flash memory is widely used since the related process is similar to that of DRAM, fine linewidths can be embodied due to simple structure, and it is a non-volatile memory. However, it is associated with the problems of increased difficulty in processing and deterioration of electrical properties beyond sub-30 nm as well as large power consumption and low operation speed due to the intrinsic high operation voltage.
As a new memory device to solve these problems, a resistive random-access memory (ReRAM), which is a low-power, highly-integrated, non-volatile memory capable of fast operation, is actively studied. The ReRAM is a memory using a material whose resistance changes under bias sweep. According to the resistive switching behavior, it is divided into the unipolar switching type and the bipolar switching type. Although the exact mechanism of each switching behavior is not clearly understood yet, it is known that unipolar switching is accomplished by the formation and extinction of a conducting path in the oxide film caused by dielectric breakdown, and bipolar switching is mainly explained by the change in Schottky barrier height or the formation of oxygen-deficient phase due to oxygen vacancy in the oxide film.
Recently, several groups are performing researches to provide transparency and flexibility to the ReRAM. The most studied flexible ReRAM is one obtained by depositing transition metal oxide such as TiO2, ZnO, etc. thinly on a flexible substrate. Although the flexible ReRAM prepared using the transition metal oxide has good resistance ratio, endurance and uniformity, it achieves flexibility simply by thinly depositing an oxide film rather than improving the intrinsic property of the oxide film. Accordingly, it does not make a fundamental solution. In addition, transparency cannot be ensured by using metal oxide.